The invention relates to a cylinder head gasket for a multicylinder engine comprising a chain case, with an engine block, a cylinder head and a chain case part which is mounted on the engine after the cylinder head gasket, which also serves to seal the chain case, is clamped between engine block and cylinder head, the cylinder head gasket having an elongate gasket plate defining a plane, and the engine having guide means for guiding the chain case part in the course of assembly thereof for displacement in the longitudinal direction of the gasket plate parallel to the plane of the gasket plate. The cylinder head gasket to which the invention relates comprises a group of combustion chamber openings, a chain case opening arranged between this group and a narrow side of the gasket plate, a gasket plate edge strip arranged between the chain case opening and this narrow side of the gasket plate, and screw openings for the passage of cylinder head screws therethrough, and, in particular, the gasket plate is essentially formed by one or several sheet metal layers.
With engines of the kind defined hereinabove, the chain case can be formed by a chain case top part arranged beside the cylinder head and a chain case bottom part arranged beside the engine block or crankcase. However, there are also constructions in which the top part of the chain case is formed by the cylinder head so that the latter engages over the engine block and a bottom part of the chain case, or in which the bottom part of the chain case is formed by the engine block above which the cylinder head and a top part of the chain case are arranged. In all these cases there results in at least one of the sealing surfaces between which the cylinder head gasket is clamped, and which are formed by the cylinder head and possibly a top part of the chain case and by the engine block and possibly a bottom part of the chain case, a joint between two engine components resting against each other, which in view of manufacturing tolerances can lead to sealing problems which will be explained with reference to an engine construction with a separate chain case top part and a separate chain case bottom part:
When assembling such an engine, the bottom part of the chain case is usually first screwed to the engine block, whereupon the cylinder head gasket which also serves to seal the chain case is placed on engine block and chain case bottom part and the cylinder head is then mounted and the cylinder head gasket clamped between the cylinder head and the engine block. The chain case top part is then inserted from the side onto positioning pins provided on the cylinder head and functioning as guide means and screwed through the cylinder head gasket to the chain case bottom part and tightened against the cylinder head gasket. Owing to manufacturing and assembly tolerances, there can result in the sealing surface resting against the one side of the cylinder head gasket in the area of the joint between engine block and chain case bottom part a step which results in the sealing surface area formed by the upper side of the chain case bottom part lying lower than the sealing surface area formed by the upper side of the engine block by, for example, 0.2xc2x10.2 mm, i.e., up to 0.4 mm. As the chain case top part is assembled on positioning pins and hence very exactly, the risk of formation of an appreciable step in the other sealing surface due to manufacturing tolerances is very much smaller, albeit not excludedxe2x80x94but, in any case, the height of a step possibly present in the sealing surface formed by cylinder head and chain case top part is significantly smaller than the height of the step possibly present in the other sealing surface.
In order to reliably seal off the sealing gap between the two above-mentioned sealing surfaces, also when such steps are present, and, more particularly, also around the chain case whose cavity is not exclusively formed in the chain case top part and chain case bottom part, but is partially also delimited by side surfaces of the engine block and the cylinder head, so that the plane of division defined by the two above-described joints, which lie exactly above one another in the direction perpendicular to the plane of the sealing gap, traverses the cavity of the chain case, it is customary, with cylinder head gaskets whose gasket plate is formed by one or several sheet metal layers, to seal around the cavity of the chain case with at least one resiliently elastic bead which defines on both main surfaces of the cylinder head gasket around the chain case cavity at least one line closed within itself, along which an increased specific surface pressure results between the cylinder head gasket and the two sealing surfaces delimiting the sealing gap. With a cylinder head gasket whose gasket plate has only a single sheet metal layer consisting of resiliently elastic material, the bead can, for example, be a so-called full bead of approximately U-shaped cross section, whose bead crest forms on the one side of the cylinder head gasket a first such line and whose two so-called bead feet form on the other gasket side two such lines. However, a so-called half bead is also possible, whose cross section corresponds to an inclined step or a Z pulled flat, and which then forms such a line on both sides of the gasket. With cylinder head gaskets whose gasket plate comprises several sheet metal layers lying one on top of the other, the beads are formed in those sheet metal layers which form the two main surfaces of the gasket plate.
In addition to such beads, one has provided cylinder head gaskets for such engines in the area of the above-described steps or plane of division on both sides of the chain case cavity with elastomeric sealing elements in order to bridge the steps in the sealing surfaces and to seal off reliably the area of these steps (see, for example, DE-C-43 37 758 and EP-B-0 701 051).
With the engines in question, which comprise a chain case, the cylinder head gasket is traversed in the area of engine block and cylinder head in the vicinity of the chain case cavity by cylinder head screws serving to clamp the cylinder head gasket. The chain case cavity usually has in cross section in the plane of the sealing gap to be sealed off by the cylinder head gasket an approximately rectangular shape, with the longitudinal sides of this rectangle extending approximately transversely to the longitudinal direction of the elongate gasket plate of the cylinder head gasket, and the bead or beads surrounding the chain case cavity extending between the chain case opening of the cylinder head gasket and its screw openings for the cylinder head screws adjacent to the chain case cavity. If, following assembly of the chain case bottom part (but prior to assembly of the chain case top part) the cylinder head gasket is clamped between engine block and cylinder head by tightening the cylinder head screws, the gasket plate of the cylinder head gasket tends to have distortions as a result of the elastic deformation of the above-described bead or beads, with the result that a center portion of a gasket plate edge strip lying between the chain case opening and an outer narrow side edge of the gasket plate tends to arch in the direction towards the space to be accommodated by the chain case top part which has still to be mounted. When the chain case top part is then inserted from the side onto the positioning pins provided on the cylinder head, there is the danger that the arched area of the said gasket plate edge strip will be damaged and the sealing ability of the cylinder head gasket thereby impaired. The same applies accordingly to cylinder head gaskets for engines which do not have a separate chain case top part and a separate chain case bottom part, but only have a separate chain case part, which after the clamping of the cylinder head gasket is then inserted from the side onto guide means, such as positioning pins on the engine block or on the cylinder head.
The invention thus relates to a cylinder head gasket of the type referred to in the first paragraph of this specification, and the object of the invention is to eliminate or at least minimize in such cylinder head gaskets the danger of damage by a chain case part which is to be inserted from the side.
This object is accomplished, in accordance with the invention, in that the outer edge area of the center edge strip portion of the gasket plate described hereinabove is bent out of the plane of the gasket plate in a direction which points away from the separate chain case part which is to be mounted subsequently. The danger is thereby eliminated or at least minimized that when inserting the chain case part which has still to be mounted after the cylinder head gasket has been clamped between engine block and cylinder head, a collision will occur between the cylinder head gasket and this chain case part, thereby causing damage to the cylinder head gasket.
Advantageous further developments of the cylinder head gasket according to the invention are to be found in claims 2 to 4.